The term broadband covers a host of new products, technologies, services, and networks. One way to define broadband networks is to categorize them as those networks that support services requiring bit rates well above one megabit per second. Business and residential subscribers will be connected to broadband networks via a common access, operating at 150 megabits per second or above, that can handle a range of different broadband service types. ATM (asynchronous transfer mode) has been chosen as the communication principle on which broadband networks will be based. A future broadband ISDN (integrated services digital network) will offer the flexibility needed to handle diverse services ranging from basic telephone service to high speed data transfer, videotelephony, and high quality television distribution. The key to this flexibility is ATM which carries digital information in special cells. This allows the network to be used efficiently by applications and services with widely differing bandwidth requirements and call characteristics.
Priority-based systems have been designed for switching ATM cells or performing other packet switching functions. In such systems, all cells (packets) for a given priority are transmitted to their destination before the first cell of the next lower priority. Within a given priority, cells are transmitted on a first come-first serve basis. All equal priority cells arriving at a given cell time are transmitted before the same priority cells arriving at the next cell time are started. This becomes the root cause of jitter because one high bandwidth call could have a cell arriving every other cell time; all of a sudden, ten or twenty cells from other calls and having the same priority as the one high bandwidth call arrive in one of the open cell times. All ten or twenty of these cells will be transmitted before the next cell of the high bandwidth call causing an absolute jitter of ten or twenty cell times. During this interval, cells for the high bandwidth call keep arriving every other cell time and are queued up. When the high bandwidth call does begin transmitting its cells again, they will be transmitted every cell time, back-to-back, until the queue is empty. Not only is such jitter unacceptable in many applications, substantial resources are required for buffering and receiver buildout delay.
In addition to being important in constant bit rate applications, jitter is also an important parameter for statistical (bursty) traffic. Variable bit rate video requires very low delay, has very high bandwidth and relatively low burstiness. Variable bit rate voice also requires moderately low delay, has relatively low bandwidth and low burstiness. Both of these applications must have relatively low jitter to guarantee that cells do not arrive "too late" and the buffer at the receiving end does not become empty. Even though this is statistical traffic, it is jitter sensitive.
Other applications, including file transfers and screen image dumps, do not typically require low delay. Such applications can have either low or high average bandwidth; however, they are very bursty. Such applications could significantly interrupt the delay sensitive applications resulting in excessive queuing delays and very large buffer buildouts on the receiving end.
In view of the foregoing, a recognized need in the art exists for an arrangement usable in ATM or other packet switching systems which will bound jitter to an acceptable amount and which is applicable to both constant bit rate and statistical traffic.